U.S. patent number 7,859,814 [Application Number 11/865,138] was granted by the patent office on 2010-12-28 for linear low capacitance overvoltage protection circuit using a blocking diode.
This patent grant is currently assigned to Littelfuse, Inc.. Invention is credited to Kelly C. Casey.
United States Patent |
7,859,814 |
Casey |
December 28, 2010 |
Linear low capacitance overvoltage protection circuit using a
blocking diode
Abstract
A low capacitance overvoltage protection circuit (80) provides
protection to a communication line (12, 14). A diode bridge (46) is
connected to the communication line (12, 14) so that overvoltages
of both polarities pass through an overvoltage protection device
(44) in one direction. A bias voltage supply 48 applies a bias
voltage across a semiconductor overvoltage protection device (44)
through isolation resistors (64, 66) to make the capacitance of the
device (44) independent of changes in communication line voltages.
When line voltages exceed the magnitude of the bias voltage, a
blocking diode (82) prevents current from flowing through the bias
voltage supply (48) in a reverse direction.
Inventors: |
Casey; Kelly C. (Flower Mound,
TX) |
Assignee: |
Littelfuse, Inc. (Des Plaines,
IL)
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Family
ID: |
39198619 |
Appl.
No.: |
11/865,138 |
Filed: |
October 1, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080094766 A1 |
Apr 24, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60852895 |
Oct 19, 2006 |
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Current U.S.
Class: |
361/119 |
Current CPC
Class: |
H02H
9/041 (20130101) |
Current International
Class: |
H02H
9/00 (20060101) |
Field of
Search: |
;361/119 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Office Action in U.S. Appl. No. 11/254,164, dated Jan. 28, 2008.
cited by other .
Office Action in U.S. Appl. No. 11/254,163, dated Jan. 22, 2008.
cited by other .
Office Action in U.S. Appl. No. 11/254,162, dated Apr. 08, 2008.
cited by other .
Jon Schleisner, High Speed Data Line Protection, Applicaiotn Notes,
Jul. 18, 2002, 3 pages. cited by other.
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Primary Examiner: Jackson; Stephen W
Attorney, Agent or Firm: Kacvinsky Daisak, PLLC
Parent Case Text
RELATED APPLICATIONS
This non-provisional patent application claims the benefit of
pending provisional application entitled "Linear Low Capacitance
Overvoltage Protection Circuit Using Blocking Diode", filed Oct.
19, 2006, Ser. No. 60/852,895. The entire disclosure of the
provisional application is incorporated herein by reference. This
application is related to pending application Ser. No. 11,254,162,
filed Oct. 19, 2005.
Claims
What is claimed is:
1. An overvoltage protection circuit, comprising: a diode bridge;
an overvoltage protection device connected between opposite nodes
of said diode bridge; a terminal of said overvoltage protection
circuit adapted for connection to a bias voltage to linearize a
capacitance characteristics of said overvoltage protection device;
and at least one blocking diode connected between the bias voltage
terminal and said overvoltage protection device to prevent current
from flowing in a reverse direction through a bias voltage supply
providing said bias voltage.
2. The overvoltage protection circuit of claim 1, wherein said
overvoltage protection circuit is adapted for connection to a
communication line, and wherein said blocking diode is connected so
that communication line voltages less than a breakover voltage of
said overvoltage protection device and greater than a magnitude of
said bias voltage prevents current from flowing into the bias
voltage supply.
3. The overvoltage protection circuit of claim 1, further including
an isolation resistor connected between the bias voltage terminal
and said overvoltage protection device terminal.
4. The overvoltage protection circuit of claim 3, wherein the bias
voltage supply includes a positive terminal connected to said
overvoltage protection device through said isolation resistor, and
said bias voltage supply has a negative terminal connected to a
second terminal of said overvoltage protection device through a
second isolation resistor.
5. The overvoltage protection circuit of claim 1, wherein said
diode bridge includes at least four diodes, not including said
blocking diode.
6. The overvoltage protection circuit of claim 1, wherein said
diode bridge includes six diodes, not including said blocking
diode.
7. The overvoltage protection circuit of claim 1, wherein said
overvoltage protection device is connected between opposite nodes
of said diode bridge, and said bias voltage is coupled between said
opposite nodes.
8. The overvoltage protection circuit of claim 7, wherein a second
node of said diode bridge is adapted for connection to a tip
conductor of a communication line, and a third node of said diode
bridge is adapted for connection to a ring conductor of said
communication line.
9. The overvoltage protection circuit of claim 8, wherein a fourth
node of said diode bridge is adapted for connection to a circuit
ground.
10. The overvoltage protection circuit of claim 1, wherein said
overvoltage protection circuit is adapted for connection to a
communication line, and wherein said bias voltage is not applied to
the communication line.
11. The overvoltage protection circuit of claim 1, further
including a packaged module to which components of said overvoltage
protection circuit are mounted, said packaged module having a
positive bias voltage terminal adapted for connection to a positive
voltage of a bias voltage supply, a negative bias voltage terminal
adapted for connection to a negative voltage of the bias voltage
supply, a tip terminal adapted for connection to a tip conductor of
a communication line, a ring terminal adapted for connection to a
ring conductor of the communication line, and a ground terminal
adapted for connection to a reference voltage.
12. The overvoltage protection circuit of claim 1, further
including two blocking diodes, a first blocking diode arranged to
prevent current from flowing into a positive terminal of the bias
voltage supply, and a second blocking diode arranged to prevent
current from flowing out of a negative terminal of the bias voltage
supply.
13. An overvoltage protection circuit, comprising: an overvoltage
protection device responsive to an overvoltage for being driven
into a low voltage conduction state; a conductor for coupling a DC
bias voltage to said overvoltage protection device to lower a
capacitance thereof; and at least one blocking diode in said bias
voltage conductor, said blocking diode forward biased by said DC
bias voltage.
14. The overvoltage protection circuit of claim 12, further
including a bridge rectifier to which said overvoltage protection
device is connected so that currents resulting from the overvoltage
are carried through said overvoltage protection device in one
direction.
15. The overvoltage protection circuit of claim 13, further
including an isolation resistor for isolating a bias voltage supply
from said overvoltage protection device.
16. An overvoltage protection circuit, comprising: a bridge
rectifier having at least four diodes, a first and second diode of
said bridge having cathodes thereof connected to a first node, and
a third and fourth diode of said bridge having anodes thereof
connected to a second node; an anode of said first diode and a
cathode of said third diode connected to a third node of said
bridge, wherein said third node of said bridge is adapted for
connection to a first communication line conductor, and an anode of
said second diode and a cathode of said fourth diode connected to a
fourth node of said bridge, wherein said fourth node of said bridge
is adapted for connection to a second communication line conductor;
an overvoltage protection device connected between said first and
second nodes of said bridge; a first blocking diode connected in
series with a first isolation resistor to form a junction
therebetween and first and second terminals, the first terminal of
said series connected first blocking diode and first isolation
resistor connected to the first node of said diode bridge, where
said first blocking diode is arranged to allow current to flow into
said first node of said diode bridge, and said second terminal of
said series connected first blocking diode and first isolation
resistor adapted for connection to a first terminal of a bias
voltage supply; and a second blocking diode connected in series
with a second isolation resistor to form a junction therebetween
and third and fourth terminals, the third terminal of said series
connected second blocking diode and second isolation resistor
connected to the second node of said diode bridge, where said
second blocking diode is arranged to allow current to flow out of
said second node of said diode bridge, and said fourth terminal of
said series connected second blocking diode and second isolation
resistor adapted for connection to a second terminal of the bias
voltage supply.
17. The overvoltage protection circuit of claim 16, further
including; a fifth diode of said bridge, a cathode of said fifth
bridge diode connected to said first node of said diode bridge; a
sixth diode of said bridge, an anode of said sixth bridge diode
connected to the second node of said diode bridge; and an anode of
said fifth bridge diode adapted for connection to a reference
voltage, and a cathode of said sixth bridge diode adapted for
connection to the reference voltage.
18. The overvoltage protection circuit of claim 17, wherein said
reference voltage comprises a ground potential.
19. A method of protecting a communication line from overvoltages,
comprising the steps of: using a thyristor to provide overvoltage
protection to a communication line; biasing the thyristor with a
bias voltage to linearize the capacitance of the thyristor and
reduce changes in capacitance with changes in voltages carried by
the communication line; and preventing current from flowing in a
reverse direction through a supply of the bias voltage to further
linearize the capacitive loading of the thyristor when the voltage
on the communication line exceeds the magnitude of the bias supply
voltage.
20. The method of claim 19, further including using a
unidirectional conducting thyristor and a diode bridge, where the
bridge diodes are connected to the communication line to allow said
thyristor to conduct in one direction in response to overvoltages
of both polarities.
21. The method of claim 19, further including biasing said
thyristor with a bias voltage to make the capacitance of said
thyristor more independent of changes in voltage and temperature to
which said thyristor is subjected.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to overvoltage protection
devices and circuits, and more particularly to overvoltage
protection circuits providing low capacitance protection to
communication lines.
BACKGROUND OF THE INVENTION
Many circuits in electronic equipment provide protection from the
harmful effects of overvoltages, overcurrents, etc. These
protection circuits are often designed as an integral part of the
general electronic circuit, but may be added thereto as ancillary
devices or circuits.
Protection circuits may often be constructed on silicon substrates,
such as bipolar transistors, diodes or thyristors. Silicon bipolar
devices can carry large magnitudes of current and thus are well
adapted for use in protecting electronic circuits from damage by
overvoltages and overcurrents. Solid state bipolar devices
constructed with junctions have an inherent capacitance that is a
function of the width of the depletion region. The depletion region
in a semiconductor junction functions as the "dielectric" layer of
a capacitor. Since the width of the depletion region varies with
the voltage impressed across the junction, the capacitance of a
bipolar semiconductor junction varies as a function of the voltage
applied across the junction. Capacitors whose values vary with
voltage are inherently nonlinear devices. In other words, a bipolar
overvoltage protection device placed across a circuit to be
protected can affect the operation of the circuit even if the
overvoltage protection device remains in its off state. The
non-linearity can lead to suboptimal channel performance and
intermodulation distortion.
The adverse affects of the foregoing are experienced in many
applications, including communication lines where overvoltage
protection circuits are routinely employed to protect transmitting
and receiving circuits from high voltages that may be inadvertently
coupled to the communication lines. Many devices in the thyristor
family can be employed to respond to the overvoltage condition and
provide a low impedance path between the communication line and
ground, or other path where the energy is safely dissipated.
The adverse affects of the use of silicon bipolar overvoltage
protection devices may not arise from the fact that such devices
have an inherent capacitance, but rather from the characteristic
that the capacitance changes as a function of the voltage,
frequency and temperature to which the device is subjected. As an
example, many communication lines are adapted for carrying high
speed digital signals of various protocols, including ADSL, T1, E1,
ADSL2+, ADSL2++, 10BaseT, VDSL, VDSL2, T3, 100BaseT and others.
Many of these protocols are carried between remote destinations by
way of modems or other transmission and receiving circuits. In
order to optimize the transmission of high speed data, many modems
utilize an initial process of selecting the proper equalization
components so that the digital signals can be transmitted at the
highest speed permitted by the frequency response of the line and
the circuits associated with the line. The equalization parameters
selected by the modem are those that exist at the time equalization
testing is carried out. This is usually once when the modem is
placed in service, and on each reboot thereof after initial
operation. It can be seen that if the electrical state of the line
changes after the equalization session, the transmission data rate
may not be optimized, and thus transmission errors can occur.
An example of transmission inefficiencies can arise in connection
with the following example. A modem placed on line or booted into
operation will be programmed to automatically carry out an
equalization process for determining the best electrical parameters
to be switched into operation to optimize high speed data
transmission. The modem will be connected to the communication
line, such as a telephone DSL line adapted for carrying VDSL or
other data signals. An on-hook state (of the telephone set) of the
DSL line for carrying digital signals is typically 48 volts. After
the modem has completed the equalization process, it is situated to
provide optimum transmission of the VDSL signals, based on the
electrical characteristics of the DSL communication line that
existed during the equalization process. Typically, the modem will
adapt the voltage magnitude of the digital signals as a function of
the length of the communication line so that the lowest power level
is achieved while yet minimizing the transmission data error
rate.
During an actual communication session by a user in which the VDSL
signals are being transmitted at a high rate, assume that the
user's telephone set connected to the same DSL communication line
is placed in an off-hook condition. In other words, the user is
simultaneously using the DSL communication line for both verbal
communications with the telephone set, and for data communications
using the modem. This off-hook condition places a different set of
voltages on the communication line. The communication line goes
from a 48-volt on-hook state to about a 10-volt off-hook state. As
such, the capacitance of the overvoltage protection devices, and
possibly other devices, will change with changing voltages, thus
modifying the electrical characteristics of the lines to which the
modem was equalized. With the communication line now having
different electrical characteristics, the effective transmission
rate may be lowered, but the modem keeps transmitting at the rate
optimized during the equalization session. As a result, the data
receiver or modem at the receiving end of the communication line
may detect errors arising from the transmission of data at a rate
higher than the line can reliably carry in the off-hook condition.
The excessive error rate may cause the modem to retrain, which
results in a temporary loss of service during the retraining
session. This is generally unacceptable and annoying to the
user.
From the foregoing, it can be seen that a need exists for a
technique for making overvoltage protection devices and circuits
less prone to changes in capacitance as a function of voltage, and
thereby reduce the change in electrical characteristics of the
devices or circuits connected to the lines. Another need exists for
an overvoltage protection circuit that includes a bias voltage
applied to the overvoltage protection device to minimize changes in
capacitance as a function of voltage applied across the device, and
in which the magnitude of the bias voltage need not be greater than
the line voltages experienced by the line to be protected. Still
another need exists for an overvoltage protection circuit in which
a bias voltage is applied to an overvoltage protection device for
minimizing the change in capacitance of the device as a function of
both voltage and frequency applied across the device, and also as a
function of the temperature of the overvoltage protection
device.
SUMMARY OF THE INVENTION
In accordance with an important feature of the overvoltage
protection circuit of the invention, a bipolar overvoltage
protection device is biased with a voltage to reduce the change in
capacitance as a function of voltage applied to the device. In
addition, at least one blocking diode is placed in series with the
bias voltage supply to prevent current from flowing therethrough in
a reverse direction. With this arrangement, when high voltages
normally occurring on the line to be protected are present, a
current flow path through the bias supply is avoided, which would
otherwise contribute to the capacitance of the circuit.
In accordance with one embodiment of the invention, disclosed is an
overvoltage protection circuit which includes a diode bridge and an
overvoltage protection device connected between opposite nodes of
the diode bridge. A terminal of the overvoltage protection circuit
is adapted for connection to a bias voltage is coupled to linearize
a capacitance characteristics of the overvoltage protection device.
At least one blocking diode is connected between the bias voltage
terminal and the overvoltage protection device to prevent current
from flowing in a reverse direction through the bias voltage supply
that provides the bias voltage.
According to another embodiment of the invention, disclosed is an
overvoltage protection circuit which includes an overvoltage
protection device responsive to an overvoltage for being driven
into a low voltage conduction state. A conductor is provided for
coupling a DC bias voltage to the overvoltage protection device to
lower a capacitance thereof. At least one blocking diode is
provided in the bias voltage conductor, and the blocking diode is
forward biased by the DC bias voltage.
According to another embodiment of the invention, discloses is an
overvoltage protection circuit which includes a bridge rectifier
having at least four diodes, a first and second diode of the bridge
having cathodes thereof connected to a first node, and a third and
fourth diode of the bridge having anodes thereof connected to a
second node. The anode of the first diode and a cathode of the
third diode are connected to a third node of the bridge, and the
third node of the bridge is adapted for connection to a first
communication line conductor. The anode of the second diode and a
cathode of the fourth diode are connected to a fourth node of the
bridge, and the fourth node of the bridge is adapted for connection
to a second communication line conductor. An overvoltage protection
device is connected between the first and second nodes of the
bridge. A first blocking diode is connected in series with a first
isolation resistor to form a junction therebetween and first and
second terminals. The first terminal of the series connected first
blocking diode and first isolation resistor are connected to the
first node of the diode bridge, where the first blocking diode is
arranged to allow current to flow into the first node of the diode
bridge. The second terminal of the series connected first blocking
diode and first isolation resistor are adapted for connection to a
first terminal of a bias voltage supply. A second blocking diode is
connected in series with a second isolation resistor to form a
junction therebetween and third and fourth terminals. The third
terminal of the series connected second blocking diode and second
isolation resistor are connected to the second node of the diode
bridge, where the second blocking diode is arranged to allow
current to flow out of the second node of the diode bridge. The
fourth terminal of the series connected second blocking diode and
second isolation resistor is adapted for connection to a second
terminal of the bias voltage supply.
According to yet another embodiment of the invention, disclosed is
a method of protecting a communication line from overvoltages. The
method includes the steps of using a thyristor to provide
overvoltage protection to a communication line, and biasing the
thyristor with a bias voltage to linearize the capacitance of the
thyristor and reduce changes in capacitance with changes in
voltages carried by the communication line. Current is prevented
from flowing in a reverse direction through a supply of the bias
voltage to further linearize the capacitive loading of the
thyristor when the voltage on the communication line exceeds the
magnitude of the bias supply voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages will become apparent from the
following and more particular description of the preferred and
other embodiments of the invention, as illustrated in the
accompanying drawings in which like reference characters generally
refer to the same parts, functions or elements throughout the
views, and in which:
FIG. 1 is a schematic drawing of an overvoltage protection circuit
which linearizes the capacitance of the thyristor device;
FIG. 2 is a graph depicting the capacitance/frequency
characteristics of a bipolar thyristor device as a function of
various bias voltages applied thereto;
FIG. 3 is graphically depicts the capacitance/line voltage
characteristics of various overvoltage protection devices, and of
the overvoltage protection circuit of FIG. 1, during different
conditions of the communication line, and without the blocking
diode of the invention;
FIG. 4 is a schematic drawing of an overvoltage protection circuit
incorporating a blocking diode;
FIG. 5 is a graph similar to that of FIG. 3, but showing the
different characteristics of the overvoltage protection circuit of
FIG. 4 when the communication line voltage exceeds the bias
voltage, and with the blocking diode of the invention;
FIG. 6 is a graph illustrating the capacitive characteristics of an
overvoltage protection circuit of the invention, with different
magnitudes of a bias voltage applied across the overvoltage
protection device;
FIG. 7 is a graph illustrating the capacitive characteristics of an
overvoltage protection circuit of the invention as a function of
temperature, for two different overvoltage protection devices;
FIG. 8 is another embodiment of the invention utilizing a grounded
bias voltage supply;
FIG. 9 is yet another embodiment of the invention employing a
four-diode bridge and a TVS device;
FIG. 10 is another overvoltage protection circuit which
incorporates a bias voltage, a blocking diode and a unidirectional
overvoltage protection device;
FIG. 11 is a diagram illustrating a module incorporating the
overvoltage protection circuit according to the invention; and
FIG. 12 is a drawing illustrating the pinout of a QFN package
embodying an overvoltage protection circuit of the invention.
DETAILED DESCRIPTION
With reference to FIG. 1, there is shown a overvoltage protection
circuit 10 adapted for protecting low voltage communication lines,
such as ADSL, T1, E1, ADSL2+, ADSL2++, 10BaseT, VDSL, VDSL2, T3,
100BaseT and others. This overvoltage protection circuit 10 is
described in detail in pending U.S. application Ser. No. 11/254,162
filed Oct. 19, 2005. This overvoltage protection circuit 10 can be
connected across a communication line, such as a telephone line
having a tip conductor 12 and a ring conductor 14. In the event
that a voltage exceeding the breakover voltage (V.sub.BO) of the
overvoltage device 44 is coupled to one or both of the
communication line conductors 12 or 14, the overvoltage device 44
will be driven into conduction through the diode bridge 46 and
short circuit the current to ground 50. The overvoltage can exist
between the communication line conductors 12 and 14, between the
conductor 12 and ground 50, between the conductor 14 and ground 50,
or between both conductors 12 and 14 and ground 50. The bridge 46
allows a unidirectional overvoltage device 44 to be employed to
couple overvoltages of either polarity to ground 50.
According to an important feature of this circuit 10, a bias
voltage 48 is applied across the overvoltage protection device 44,
which may be a unidirectional Sidactor overvoltage protection
device obtainable under the brand name Teccor, from Littelfuse,
Inc., Irving, Tex. The utilization of a bias voltage applied across
the overvoltage protection device 44 linearizes the capacitive
characteristics of the circuit 10. By this, it is meant that the
variations in capacitance (.DELTA.c) as a function of the voltage
and frequency to which the circuit 10 is subjected, is reduced. In
addition, changes in capacitance resulting from changes in the
operating temperature of the overvoltage protection device are
reduced. Accordingly, the .DELTA.c characteristics as a function of
changes in voltage, frequency and temperature are more linear and
are more independent of changes in such parameters. In gigabit
Ethernet applications, the biasing of the overvoltage protection
device has made a marked improvement in reducing errors in an
environment in which the temperature either changes, or remains at
an elevated level. Indeed, the greater the bias voltage, the less
the change in capacitance of the circuit as function of voltage,
frequency and temperature.
FIG. 2 graphically illustrates the capacitance/frequency
characteristics of a conventional bipolar overvoltage protection
device, as a function of different voltages applied across the
device. It is noted that the horizontal frequency axis is a
logarithmic scale. In particular, the graph depicts the electrical
characteristics of a Sidactor overvoltage protection device, part
number P3100SCMC, available from Teccor Electronics. Line 30
depicts the capacitive/frequency characteristics of the device with
0 volts applied thereacross. Line 32 depicts the
capacitive/frequency characteristics of the device with 1 volt
applied thereacross. Line 34 depicts the capacitive/frequency
characteristics of the device with 2 volts applied thereacross.
Line 36 depicts the capacitive/frequency characteristics of the
device with 5 volts applied thereacross. Line 38 depicts the
capacitive/frequency characteristics of the device with 15 volts
applied thereacross. Line 40 depicts the capacitive/frequency
characteristics of the device with 40 volts applied thereacross.
These electrical characteristics are those of the device while in a
test circuit, without any other communication circuits connected
thereto.
It is noted that with higher voltages applied across the bipolar
overvoltage protection device, there is less change in capacitance
as a function of frequency. This is generally true of most bipolar
overvoltage protection devices. However, the problem encountered is
that the voltage across an overvoltage protection device cannot
generally be known or predicted at all times when connected to
communication circuits or communication lines. Thus, when the
voltage across such device is low, the other communication circuits
will have to operate with the variations of capacitance of the
overvoltage protection device. When transmitting VDSL and other
high speed digital signals on a DSL line, this means either
reducing the transmission speed to a rate less than otherwise would
be necessary to accommodate the increased capacitance of the
overvoltage protection device, or accept a higher error rate.
Neither of these solutions is acceptable to either communication
providers or users.
With reference back to FIG. 2, it is noted that for low voltages
across the overvoltage protection device, namely between about one
and five volts, the capacitance of the device changes substantially
more than for higher voltages across the device. For the particular
device of the example, and for a voltage of one volt applied across
the device, the capacitance of the device changes about 5 pf, from
about 40 pf to about 35 pf, between the frequency range of 1 MHz
and 10 MHz. This represents about a 12.5% change in capacitance of
the device. With the 1-volt potential applied across the device,
the capacitance at 100 MHz is about 33 pf. Considering the same
overvoltage protection device with a voltage of 40 volts across it,
it is noted that the change in capacitance between 1 MHz and 10 MHz
is about 1 pf (28 pf-27 pf), or about a 3.6% change in capacitance.
At 100 MHz, the capacitance presented by the device with a 40 volt
bias across it, is about 24 pf. The small degree of change in
capacitance is seen from FIG. 2 by the linear 40-volt line. The
other lines in the graph representing lower voltages across the
device are much more non-linear. It can also be seen that by
assuring there is a voltage greater in magnitude than normally
applied across the device, there is a corresponding reduction in
capacitive change within the device. As noted above, with larger
voltages applied across a bipolar semiconductor junction, there is
a wider depletion region between the conductive regions (the
capacitor plates) of the device, and thus less capacitance.
In accordance with an important feature of the invention, a bias
voltage is applied to the overvoltage protection device 44 so that
it operates in a continuous manner with a lower capacitance,
thereby allowing communication lines to operate with optimal speed
and bandwidth. By assuring that there is always at least a
predetermined voltage across the overvoltage protection device, it
is assured that the communication line connected thereto undergoes
a minimal degree of change in capacitance--at least the capacitance
contributed by the overvoltage protection device. The foregoing
advantage is also realized in situations where the overvoltage
protection device experiences changes in frequency and
temperature.
FIG. 1 illustrates an overvoltage protection circuit 10 employing a
floating bias supply voltage. Here, a Sidactor overvoltage
protection device 44 is a unidirectional bipolar device that is
connected to a communication line which includes a tip conductor 12
and a ring conductor 14. The tip conductor 12 and the ring
conductor 14 are connected to nodes 68 and 69 of the overvoltage
protection device 44 by respective diode pairs of the bridge 46.
The tip conductor 12 is connected to the overvoltage protection
device 44 by way of diode pairs 52 and 54, while the ring conductor
14 is connected to the overvoltage protection device 44 by diode
pairs 56 and 58. The overvoltage protection device 44 is connected
to ground 50 via nodes 68 and 69 by way of diode pairs 60 and 62.
In particular, the cathode of diode 62 is connected to ground 50
and the anode of diode 60 is connected to ground 50. Overvoltages
of either polarity can be conducted in a conventional manner from
either the tip conductor 12 or the ring conductor 14, or both, to
ground 50 through the various diodes of the bridge 46. As can be
seen, the various current paths through the overvoltage protection
device 44 include a first diode, the overvoltage protection device
44, and then a second diode. The three components are all in
series, thus reducing the effective capacitance presented by the
overvoltage protection circuit 10 to the communication line 12 and
14.
The bias voltage of the supply 48 is applied across the terminals
of the overvoltage protection device 44. The bias voltage is
preferably a DC voltage and is applied to the overvoltage
protection device 44 on a continuous basis. Alternatively, the bias
voltage can be applied only during the time when the communication
line 12 and 14 is active in transmitting communication signals.
The bias voltage is applied to the overvoltage protection device 44
by the bias voltage supply 48, through at least one isolation
resistor 64 in the floating bias supply embodiment, and preferably
two isolation resistors, one shown as optional isolation resistor
66. The isolation resistors 64 and 66 are of sufficiently high
resistance so as to provide isolation between the bias voltage
supply 48 and the overvoltage protection device 44 when the latter
is driven into a conductive state in response to an overvoltage. In
practice, the isolation resistors 64 and 66 can be on the order of
one megohm each, or larger. However, in certain applications, the
value of each resistor 64 and 66 could be as low as several hundred
ohms. During periods of time when an overvoltage on the
communication line 12 and 14 triggers the overvoltage protection
device 44 into conduction, the presence of the bias voltage does
not otherwise affect the breakover voltage or other electrical
characteristics of the overvoltage protection device 44.
The polarity of the bias voltage supply 48 is chosen such that the
diodes of the bridge 46 are reverse biased during normal operation
of the communication line 12 and 14. While the bias voltage supply
48 is shown non-referenced to ground, i.e., floating, in other
communication line situations, the overvoltage protection circuit
can be configured differently. In the various configuration of the
bias voltage supply, it is important to note that the positive
terminal of the bias voltage supply should not be negative with
respect to ground, and the negative terminal should not be positive
with respect to ground.
In operation of the overvoltage protection circuit of FIG. 1, it is
assumed that the bias voltage of the supply 48 is a) greater than
the highest operating voltage of the communication line, and b)
lower than the breakover voltage of the overvoltage protection
device 44. As will be described below, the constraint a) is not
necessary when using a blocking diode in series with the bias
voltage supply 48. The capacitance of the overvoltage protection
device 44 remains relatively unchanged due to the constant bias
voltage applied across the device 44. Changes in line voltage
change the capacitance of the bridge diodes because the voltage
across the diodes changes with line voltage. As noted above, when
operating with high data rates or in other environments where
circuit capacitance changes are critical and undesirable, the
biasing of overvoltage protection devices 44 can improve
communication line performance and reduce data transmission
errors.
The overvoltage protection circuit 10 of FIG. 1 functions for its
intended purpose, with several considerations. First, and as noted
above, the overvoltage protection device 44 is preferably selected
with a breakover voltage greater than high voltages normally
transmitted on the communication line, such as signaling or ringing
voltages, which are generally greater in magnitude than the battery
voltage or the voice or data signals. Secondly, the magnitude of
the bias voltage 48 should be greater than any voltage normally
encountered on the communication line. If these considerations were
not heretofore adhered to, the overvoltage protection device 44
would be driven into conduction by high operating voltages other
than overvoltages inadvertently coupled to the communication line,
or the various diodes of the bridge may be forward biased even in
the absence of an overvoltage coupled to the communication
line.
The graph of FIG. 3 illustrates the capacitive characteristics of
various thyristor devices as a function of line voltage. Line 73 of
the graph illustrates the capacitance characteristics of a single
Sidactor overvoltage protection device (P2600SCMC) having a 260
volt breakover voltage. Line 75 illustrates the capacitive
characteristics of two series-connected Sidactor overvoltage
protection devices (P2703ACMC), each having a 135 volt breakover
voltage. Line 77 illustrates the capacitive characteristics of four
series-connected Sidactor overvoltage protection devices
(P0720SCMC), each having a 72 volt breakover voltage. Line 79
illustrates the capacitive characteristics of another Sidactor
overvoltage protection device (P3100SCMC) connected in series with
a pair of antiparallel MUR diodes. It is noted that the line
voltage of a VDSL2 line is represented by the horizontal axis of
FIG. 3.
When a semiconductor overvoltage protection circuit 10, such as the
type shown in FIG. 1, is coupled across a twisted pair tip and ring
type of communication line, the capacitance/line voltage
characteristics of such circuit are as shown by lines 70 and 72 in
FIG. 3. Line 70 of the graph illustrates that the capacitance of
the overvoltage protection circuit 10 is about 12 pf with zero bias
applied across the overvoltage device 44. In other words, the bias
voltage 48 shown in FIG. 1 is open circuited. When the bias voltage
48 of the device 44 in the overvoltage protection circuit 10 is
increased to about 53 volts, the capacitance/line voltage
characteristics are as shown by line 72 in FIG. 3. In this event,
it can be seen that the capacitance is lowered to about 7 pf and is
relatively linear until the communication line voltage exceeds the
53-volt bias voltage. When the line voltage on either the tip
conductor 12 or the ring conductor 14 exceeds the voltage of the
bias supply 48, the capacitance rises abruptly, as shown by rising
portion 74 of the graphical line 72. This sharp increase in the
capacitance is undesirable and can represent unstable electrical
conditions on the communication line and thus cause data
transmission errors.
The different voltages experienced across an overvoltage protection
circuit can be appreciated by noting that the most severe
conditions are at low line voltages, where the various
semiconductor overvoltage protection devices present a large
variation in capacitance to the communication line. This can occur
during dry conditions of the communication line, where the line is
effectively unpowered during times that the telephone line is
carrying concentrated traffic, as compared to an individual
telephone conversation. The line voltage during this state of the
telephone line can be between 0 volts and about 4 volts. During
off-hook conditions of the communication line, the voltage across
the tip and ring conductors can be between 8 volts and 20 volts.
Active telephone conversations are carried on the telephone line
during off-hook conditions. When a telephone set is placed in an
on-hook condition, the voltage between the tip and ring conductors
can be between 48 volts and 56 volts. These voltages are only
nominal values. In practice, the line voltages may differ
significantly from the values noted above, based on line conditions
and other circumstances. In a telephone line that is adapted for
carrying data signals, the telephone set may indeed be on hook, but
a modem may be connected to the communication line and actively
transmitting data signals.
The data transmission errors referred to above can occur when, for
example, a user of a high speed communication line is transmitting
or receiving data via a modem or other data interface, and the line
suddenly undergoes a high voltage, such as a ringing voltage or an
off-hook condition on the communication line. During those periods
when the line voltage exceeds 53 volts, in the example, the
capacitance of the overvoltage protection circuit abruptly rises.
This sudden increase in the capacitance connected across the
communication line can disrupt the electrical parameters of the
communication line to which the data modem was initially equalized
during installation or subsequent rebooting. When the communication
line exhibits a different capacitance, namely an increase in
capacitance, the electrical match between the data modem and the
communication line is upset and thus transmission errors can
occur.
Assume for purposes of example that a voltage exceeding the 53-volt
bias voltage, such as a 100-volt signal, is applied to the tip
conductor 12 of the circuit of FIG. 1. If the breakover voltage of
the overvoltage protection device 44 is greater than 100 volts,
such device 44 will remain in the non-conductive state. However,
the diode 52 of the bridge 46 will become forward biased, and
current will flow through the isolation resistor 64 and into the
bias supply 48 (in a reverse direction), and then to the ring
conductor 14 via the other isolation resistor 66, and bridge diode
58. Alternatively, the current may flow to ground from the bias
supply 48 if such supply is of the grounded type. When current
flows through the bias supply 48 in the reverse direction, the
capacitance presented to the communication line sharply increases,
as shown in the upturned line 74 of FIG. 3, and data transmission
errors can occur. It is believed that when the junction of one or
more of the bridge diodes becomes forward biased, the capacitance
of the bias voltage supply comes into play and contributes
significantly to increased capacitance of the overvoltage
protection circuit 10.
In accordance with an important feature of the invention, current
is prevented from flowing into the bias voltage supply through
forward biased bridge diodes by communication line voltages. This
is prevented by the use of a blocking diode 82a, as shown in the
overvoltage protection circuit 80 of FIG. 4. While a single
blocking diode 82a can be used, it is preferable to use a pair of
blocking diodes 82a and 82b. The circuit 80 of FIG. 4 is
essentially the same as that shown in FIG. 1, and includes many of
the same low capacitance advantages, except for the addition of the
blocking diodes 82a and 82b. The blocking diode 82a is poled so
that leakage current from the bias voltage supply 48 can pass
through the forward-biased blocking diode 82a and to the
overvoltage device 44. However, in the event of the application of
a high voltage on the communication line, such as a ringing
voltage, on either the tip conductor 12 or the ring conductor 14,
the blocking diode 82a is reverse biased. In other words, with the
overvoltage protection circuit 80 of FIG. 4, current flow into the
bias voltage supply 48 in response to a high line voltage on the
communication line is prevented as the current paths each have the
reverse-biased blocking diode 82a and 82b therein.
In a preferred embodiment of the invention shown in FIG. 4, the
overvoltage protection device 44 is connected between opposite
nodes 68 and 69 of the diode bridge 46. The isolation resistor 64
and optional isolation resistor 66 are also coupled to the
respective nodes 68 and 69, through respective blocking diodes 82a
and 82b. The isolation resistors 64 and 66 function to isolate the
bias voltage supply 48 from the diode bridge nodes 68 and 69. In
other words, when an overvoltage on the communication line drives
the overvoltage protection device 44 into conduction, the bias
voltage supply 48 is not effectively short circuited. The tip
conductor 12 of the communication line is connected to a node 20 of
the bridge 46 defined by the cathode of diode 52 and the anode of
diode 54. The ring conductor 14 of the communication line is
connected to a node 22 of the bridge 46 defined by the anode of
diode 56 and the cathode of diode 58. A circuit ground 50 or other
reference potential is connected to a node 24 of the bridge defined
by the anode of diode 60 and the cathode of diode 62.
The blocking diode 82a can be the only blocking diode employed in
the overvoltage protection circuit 80 shown in FIG. 4. In like
manner, the blocking diode 82b can be the only blocking diode
employed, and can be placed on the negative side of the bias
voltage supply 48. In this latter event, the blocking diode 82b
would be poled so that current resulting from a high voltage on the
communication line is prevented from flowing through the bridge
diodes and through the bias voltage supply 48. Preferably, both
blocking diode 82a and 82b are placed in both the positive and
negative supply lines on the bridge side of the isolation resistors
64 and 66. Although the blocking diodes 82a and 82b can be placed
on either side of the respective isolation resistors 64 and 66, it
is preferable to place the blocking diodes 82a and 82b on the
bridge side of the isolation resistors 64 and 66. The reason for
this is the blocking diodes 82a and 82b can then be fabricated in a
semiconductor chip along with the bridge diodes. The blocking
diode(s) can be placed in overvoltage protection circuits at many
locations so that undesired leakage currents do not flow through
the bias voltage supply 48 in a reverse direction. Indeed, if it is
found in other types of overvoltage protection circuits that
similar leakage current paths exist in response to high line
voltages, blocking diodes can be employed to block such currents
and maintain a relatively constant capacitance characteristic of
such overvoltage protection circuits.
FIG. 5 illustrates the capacitance/line voltage characteristics of
the overvoltage protection circuit 80 of FIG. 4. Graphical lines
73, 75, 77, 79 and 70 illustrate the capacitive characteristics of
the overvoltage protection circuit 80, without a bias voltage
applied across the overvoltage protection device 44, and with
various different configurations of overvoltage protection devices
described above. The graphical line 84 illustrates the capacitive
characteristics of the overvoltage protection circuit 80 of FIG. 4,
with a 53 volt bias applied across the overvoltage protection
device 44. The graphical line 84 is generally linear through the
voltage range of the communication line, but rises somewhat
starting with a line voltage at about 50 volts. At about 53 volts,
the line 84 continues in a manner coincident with the line 70 which
represents zero bias applied to the overvoltage protection device
44. Importantly, in the overvoltage protection circuit 80, the
capacitance is never greater than the case where a bias voltage of
zero volts (line 70) is applied across the overvoltage protection
device 44. In contrast to that of FIG. 3 (shown by line 72), the
line 84 of FIG. 5 does not increase in capacitance above the zero
bias value when a voltage greater than the bias voltage supply 48
is applied to one or both conductors 12 or 14 of the communication
line. With a relatively constant capacitance exhibited by the
overvoltage protection circuit 80, fewer data transmission errors
will be encountered.
In accordance with an important feature of the invention, it is no
longer necessary to provide a bias voltage that is greater than
line voltages normally encountered on the communication line, even
ring voltages. With the utilization of the blocking diodes 82a and
82b, the bias voltage can be much less than voltages normally
encountered on the communication line, whereupon current is
prevented from flowing into the bias voltage supply 48 which would
otherwise cause additional capacitance to be imposed on the
overvoltage protection circuit 80. With a virtually flat
capacitance loading characteristic, the overvoltage protection
circuit 80 can be incorporated with ease into many different types
and configurations of communication lines. Indeed, one or a few
generic overvoltage protection circuits can be provided for use
with a large variety of communication lines, without concern of the
magnitude of line voltages normally carried by the lines. The
design of data modems and other line interface circuits is made
much easier and is simplified when the capacitance loading
characteristics of the overvoltage protection circuit are generally
independent of signal frequencies, data rate or line voltages.
Stated another way, the same bias voltage supply (magnitude) can be
used without regard to the line voltages experienced in different
communication line applications.
FIG. 6 graphically illustrates the general capacitance
characteristics of an overvoltage protection circuit similar to
that described above in connection with FIG. 4, with different bias
voltages applied to the overvoltage protection device 44. Graphical
line 120 represents the capacitive characteristics of the circuit
when a zero bias voltage is applied across the device 44. As the
line voltage increases in a logarithmic manner, as depicted on the
horizontal axis of the graph, the capacitance decreases. This is
because the depletion region in the bipolar junction of the
overvoltage protection device 44 widens, thus increasing the
effective spacing between the capacitor plates and decreasing the
capacitance. With no bias voltage applied across the overvoltage
protection device 44, the capacitance continues to vary with line
voltage, which represents undesirable changing electrical
characteristics.
Line 122 of the graph of FIG. 6 represents the capacitive
characteristics of the overvoltage protection circuit with a 3.3
volt bias applied across the overvoltage protection circuit. Here,
the capacitive characteristics of the circuit remain essentially
constant up to about one volt, then increase up to the value
representative of a zero volt bias, and thereafter remain
consistent with the capacitive characteristics of the zero volt
bias of line 120. The upward breaks or turns of the capacitance
represented by line 122, and the other lines of the graph, are
believed to be capacitance contributed by the forward biasing of a
diode bridge by the line voltage. Line 124 is representative of th
capacitive characteristics of the circuit with a 5.0 volt bias
voltage. Line 126 represents the capacitance as a function of the
bias voltage of 12.0 volts. Lines 128, 130 and 132 represent the
capacitance when the bias voltage is respectively 24.0, 30.0 and
50.0 volts. As can be seen, with bias voltages of greater
magnitude, te capacitance remains relatively flat and constant over
a wider line voltage range. When employing the blocking diode(s) in
the bias circuit, there is no instance in which the capacitance
increase above that when no bias voltage is utilized at all.
FIG. 7 is a graph which depicts the capacitance characteristics of
an overvoltage protection device used with the invention, as a
function of temperature. The solid line 140 is the capacitive
characteristics of the circuit using a P3002SB Sidactor overvoltage
protection device obtainable under the brand name Teccor, from
Littlefuse, Inc. The line 140 is the capacitive characteristics
with a zero bias applied to the Sidactor device. Broken line 142
also depicts the capacitive characteristics of such device as a
function of temperature, but with a 15.0 volt bias employed. It can
be seen that when a bias voltage is applied to the overvoltage
protection device, the change in capacitance is reduced and is
relatively independent of changes in temperature. Solid line 144
and broken line 146 represent the respective capacitive
characteristics of an overvoltage protection circuit of the
invention using a Sidactor P3100SCMC overvoltage protection device.
Line 144 depicts the capacitive characteristics as a function of
temperature with zero bias applied to the Sidactor device. Line 146
depicts the capacitive characteristics as a function of temperature
with a 15.0 volt bias applied to the Sidactor device. Again, when a
bias voltage is applied to the overvoltage protection device, the
capacitance varies much less with changes in temperature. This
feature of the invention has been found to be advantageous in a
number of areas, including the gigabit Ethernet area where the use
of a bias voltage has been found to substantially reduce the
transmission errors as a function of changes in temperature.
As noted above, the overvoltage protection circuit of the invention
need not operate with a floating bias voltage supply. Rather, a
ground referenced bias voltage supply can be employed, as shown in
FIG. 8. Here, the negative terminal of the bias voltage supply 48
is grounded. Similarly, the isolation resistor 66 is also grounded
and placed in series with the second blocking diode 82b. In order
to maintain an isolated circuit, the second isolation resistor 66
is necessary. In all other respects, the overvoltage protection
circuit of FIG. 6 provides the same advantages as that described
above.
FIG. 9 illustrates an overvoltage protection circuit 76 employing a
four-diode bridge. The overvoltage protection circuit 76 is well
adapted for minimizing capacitive loading of the communication
line. This protection circuit 76 includes a bridge constructed with
four diodes with nodes 78 and 80 connected respectively to the tip
conductor 12 and the ring conductor 14 of the communication line.
Nodes 82 and 84 of the diode bridge are connected to respective
blocking diodes 82a and 82b. The isolation resistors 64 and 66 are
connected in series with the respective blocking diodes 82a and
82b, and coupled to the bias voltage supply 48 for providing a bias
voltage across a threshold device 86, such as a TVS device 86,
which could be a Zener diode or a host of other threshold devices.
The TVS device 86 can be selected to provide a reverse breakdown
voltage suitable for the application involved. The bias voltage
provides the same function as described above in connection with
the FIG. 4 embodiment above. In addition, the diodes 52, 54, 56 and
58 of the bridge can be either constructed or selected for low
capacitance, i.e., can include junction areas that accommodate only
100%-150% of the surge current capability of the overvoltage
protection device utilized. In other words, the capacitance of the
bridge diodes is minimized by controlling the current carrying
capability thereof, and not making the diode current capability
larger than necessary.
While the overvoltage protection circuit 80 shown in FIG. 4 may
provide particular advantages, the features of the invention are
not limited to such overvoltage protection circuit. FIG. 10
illustrates another application for the utilization of the
invention. Here, unidirectional protection is provided to the tip
and ring conductors 12 and 14 by an overvoltage protection device
90, as some communication line circuits require only such type of
protection. The positive terminal of the bias voltage supply 48 is
connected through the isolation resistor 64 to the anode of the
blocking diode 82a. The cathode of the blocking diode 82a is
connected to one terminal of a unidirectional overvoltage
protection device 90. In like manner, the negative terminal of the
bias voltage supply 48 is connected to the isolation resistor 66
which, in turn, is connected to the cathode of the second blocking
diode 82b. The anode of the blocking diode 82b is connected to the
other terminal of the unidirectional overvoltage protection device
90. The cathode of a diode 94 is connected to the cathode of the
blocking diode 82a, and the anode of a diode 96 is connected to the
anode of the other blocking diode 82b. The anode of the diode 94 is
connected to the cathode of a diode 92, as well as connected to the
ring conductor 12. The cathode of the diode 96 is connected to the
anode of the diode 92, as well as to the ring conductor 14.
As can be appreciated, the unidirectional overvoltage protection
device 90 is biased with a bias voltage 48 to linearize the
capacitance characteristics thereof. The isolation resistors 64 and
66, as well as the blocking diodes 82a and 82b, provide the same
functions described above. The diodes 94 and 96 prevent the bias
voltage 48 from biasing the tip conductor 12 or the ring conductor
14. In operation, when a positive overvoltage is applied to the tip
conductor 12, the overvoltage protection device 90 breaks down and
conducts the resulting current. The current resulting form the
overvoltage passes through the diode 94, the overvoltage protection
device 90 the diode 96 and to the tip conductor 14. In the event of
a positive overvoltage on the tip conductor 14, the resulting
current passes through the diode 92 to the tip conductor 12. Again,
the magnitude of the bias voltage supply 48 can be smaller than
communication line voltages normally encountered on such line.
In the implementation of the blocking diode or diodes, those
skilled in the art may find it expedient to fabricate a
semiconductor chip that incorporates the blocking diodes 82a and
82b together on the same chip with one or more of the bridge
diodes. For example, the blocking diode 82a of the FIG. 4
embodiment could be fabricated in the same chip as used to
fabricate the bridge diodes 52, 56 and 60, with all four diode
cathodes connected in common. In this event, the blocking diode 82a
would be connected between the bridge node 68 and the isolation
resistor 64. In like manner, the blocking diode 82b can placed in
the negative bias voltage supply circuit and be fabricated in the
same chip as the bridge diodes 54, 58 and 62, where all of the
diode anodes are connected in common. In this event, the blocking
diode would be connected between the anodes of the bridge diodes
54, 58 and 62, and the isolation resistor 66. The bridge diodes 60
and 62 are preferably constructed so as to have a current carrying
capability about twice that of the other bridge diodes. This is
because each diode 60 or 62 may be required to carry currents
resulting from overvoltages occurring simultaneously on both tip
and ring conductors 12 and 14.
In accordance with the concepts of the invention, the various
embodiments of the overvoltage protection circuits provide enhanced
and superior performance over the protection circuits known in the
art. A few guidelines in biasing the circuits according to the
invention can be set forth as follows. The positive voltage of the
bias voltage should preferably not be less than zero volts. The
negative voltage of the bias voltage should preferably not be
greater than zero volts. Lastly, the bias voltage should be less
than the breakover or threshold voltage of the overvoltage
protection device used. It should be appreciated that these are
only guidelines, as there may be situations where the principles
and concepts of the invention can be employed without abiding by
some or all of the guidelines. As can be appreciated, the practice
of the invention no longer requires that the bias voltage be
greater than the communication line voltage.
FIG. 11 illustrates an embodiment of the overvoltage protection
circuit 120 packaged in a module. FIG. 12 shows the pinout of the
QFN package 100, and also illustrates the conductor paths and
conductor pads to which the components of the overvoltage
protection circuit 120 are connected. The module is a QFN surface
mount package 100, well adapted for use with telephone line
interface units, and other communication lines. As can be seen, the
tip and ring terminals of the overvoltage protection circuit are
bridged across the respective communication line conductors 12 and
14. The ground conductor is connected to the communication line
circuit ground. The communication line illustrated includes a pair
of fuses 101 and 103 to provide overcurrent protection to the
respective conductors 12 and 44 and circuits connected thereto.
Other overcurrent protection circuits and devices can be employed,
including positive temperature coefficient resistors. In the
example, the communication line is connected to the primary of a
line transformer 105. The secondary of the line transformer 105 is
connected to a chipset 107 adapted for processing digital signals
carried by the communication line. The chipset can be a DSL driver
of the type processing DSL signals. Of course, the various
overvoltage protection circuits of the invention can be coupled to
a host of other communication lines with chipsets adapted for
carrying many other digital signal protocols and formats.
The embodiment of the overvoltage protection circuit illustrated in
FIG. 11 employs an external floating bias supply 48 coupled to the
package 100 by way of external dual isolation resistors 64 and 66.
The positive terminal of the bias voltage supply 48 is connected to
the isolation resistor 64, and the negative terminal of the bias
voltage supply 48 is connected to the isolation resistor 66. The
other terminal of the isolation resistor 64 is connected to pin 7
of the package 100, and the other terminal of the isolation
resistor 66 is connected to pin 2 of the package 100. Package
terminals 1 and 8 are both connected to the communication line tip
conductor 12. Package terminals 4 and 5 are both connected to the
communication line ring conductor 14, and package terminals 3 and 6
are both connected to circuit ground.
The overvoltage protection circuit 120 of FIG. 11 includes a
six-diode bridge 46, two blocking diodes 82a and 82b, and a
Sidactor overvoltage protection device 44 obtainable from
Littelfuse, Inc., Des Plaines, Ill., under the Teccor brand name.
The Sidactor overvoltage protection device 44 can be selected to
provide breakover voltages as low as 5-8 volts for Ethernet
applications, 30 volts for dry loop applications, and 250-350 volts
for DSL-over-POTS applications. According to one embodiment of the
invention, the circuits and devices defining the overvoltage
protection circuit include one chip fabricated with four diodes
therein, namely diodes 52, 56, 60 and 82a. A second chip is
fabricated to incorporate therein diodes 54, 58, 62 and 82b. A
third chip includes the overvoltage protection device 44. The three
semiconductor chips are interconnected to form the circuit shown in
the outline of package 100 of FIG. 9.
The package 100 is fabricated with various metallic conductor paths
and pads, all shown in FIG. 10. The package 100 is constructed with
a ring conductor path 102, a large ground conductor path 104, a tip
conductor path 106, a first isolation resistor conductor pad 108
and a second isolation resistor conductor pad 110. The QFN package
100 is preferably an 8-pin package. As noted above, the overvoltage
protection circuit 120 can be easily implemented with a
communication line interface without breaking or otherwise
interrupting any conductor paths. Rather, the overvoltage
protection circuit 120 is simply bridged across the existing tip
conductor, ring conductor and ground conductor circuits of the line
interface. In like manner, the overvoltage protection circuit 120
can be easily incorporated into new designs of communication line
interface circuits. The ring conductor path 102 and the tip
conductor path 106 are constructed as heavy metallic paths so that
such paths can be inserted in series with existing ring and tip
conductors without affecting the current-carrying capability of
such conductors.
Node 22 of the diode bridge 46 is connected to the ring conductor
path 102 which defines pins 4 and 5 of the package 100. Node 20 of
the diode bridge 46 is connected to the tip conductor path 106
which defines pins 1 and 8 of the package 100. Node 24 of the diode
bridge 46 is connected to the ground conductor path 104 which
defines pins 3 and 6 of the package 100. One terminal of the
customer-provided isolation resistor 64 is connected to the
isolation resistor conductor pad 108 which defines pin 7 of the
package 100. One terminal of the other isolation resistor 66 is
connected to resistor conductor pad 110 which defines pin 2 of the
package. As noted above, the isolation resistors 64 and 66 are
preferably of a large value to provide electrical isolation between
the bias voltage supply 48 and the overvoltage protection device 44
when the latter is driven into conduction.
The bias voltage supply 48 can be of any conventional type,
including a regulated or unregulated reference supply. Bias voltage
supplies of various voltages can be employed, depending on the
application involved. As noted above, when the principles of the
invention are employed, the voltage of the bias supply 48 need not
be greater than the high voltages normally encountered on the
communication line. The external bias voltage supply 46 is employed
to bias the overvoltage protection device 44 to achieve a low
capacitance circuit 10 well adapted for use with high speed digital
communication lines. The positive terminal of the bias voltage
supply 48 is connected to the terminal of isolation resistor 64.
The negative terminal of the bias voltage supply 48 is connected to
second isolation resistor 66.
One embodiment is described in connection with its integration into
a QFN package. However, many other types of packages, including
circuits with discrete components mounted on printed circuit
boards, can be realized. Similarly, other packages with other pin
configurations are readily achievable. Similarly, those skilled in
the art can integrate all of the solid state components into a
single chip, including the overvoltage protection device. Further,
the entire overvoltage protection circuit, including the isolation
resistors, can be incorporated into a single module. Even the bias
voltage supply components can be incorporated into the same module
as the overvoltage protection circuit.
While the foregoing overvoltage protection circuits are described
in connection with the use of a Sidactor overvoltage protection
device, other thyristor devices can be employed, whether gated or
ungated. In addition, threshold devices, such as Zener diodes and
TVS devices constructed with bipolar semiconductor technology, and
other bipolar threshold devices, can be used with the invention and
achieve reduced operating capacitance. The preferred embodiments
described herein employ a communication line having a pair of
conductors. This is not a restriction of the practice of the
invention, as the concepts and principles can be applied to a
communication line having a single conductor.
While the preferred and other embodiments of the invention have
been disclosed with reference to specific overvoltage protection
circuits, it is to be understood that many changes in detail may be
made as a matter of engineering choices without departing from the
spirit and scope of the invention, as defined by the appended
claims.
* * * * *